Climate has always changed naturally, and this is not good news when contemplating a human-forced future. The natural responses have been as large as, or larger than, those simulated by leading models for shorter time scales, with major biological and physical impacts. The possible effects of rapid carbon dioxide (CO2) release may be clearest from the Paleocene-Eocene Thermal Maximum (PETM) about 55.9 million years ago, when a large, natural CO2 release drove strong warming that caused amplifying feedbacks, dwarfing of large animals, ecosystem disruptions, soil degradation, water-cycle shifts, and other major changes (see the figure). The climatic changes during the PETM occurred over longer time scales than those of anthropogenic climate change. The impacts of the latter may thus be even more severe.